For a label or tape constructions, a product which passes a Tappi Useful Method 204 (Tappi UM 204), a de-inking and repulping test incorporated herein by reference will be considered repulpable.
Most rubber-based and acrylic adhesive systems are not repulpable. During the repulping process (fiber shearing), the adhesives tend to agglomerate and form globules, commonly referred to in the trade as "stickies." Stickies adversely affect paper quality and have a negative impact on the process of recycling paper.
Several approaches can be taken to remove adhesive stickies. One has been to use water-dispersible adhesive microspheres prepared by processes described in U.S. Pat. No. 3,691,140 to Silver, U.S. Pat. No. 4,155,152 to Baker et al., U.S. Pat. Nos. 4,495,318 and 4,598,212 to Howard, and U.S. Pat. No. 4,810,763 to Mallya et al., each incorporated herein by reference, and applied at appropriately low coat weights. While the adhesives disperse in the paper fiber adequately during the repulping process, the adhesive is not removed from the paper fibers. Rather, the adhesive microspheres accumulate in the paper fibers, reducing paper quality and limit the number of times the paper fibers can be repulped or recycled.
Water-soluble adhesive systems which provide for complete adhesive removal from the fiber have been used commercially. Typically, commercial water-soluble products have suffered from (a) poor shelf life (the tape or label is sold in a sealed plastic bag); (b) poor humidity and/or heat-aged performance; and (c) a propensity to bleed into the paper facestock (which reduces peel performance). Each of these problems is related to the high water-sensitivity of the respective products. Such water-soluble adhesives and tapes are described, for instance, in U.S. Patent 3,441,430 to Peterson; U.S. Pat. No. 3,865,770 to Blake, U.S. Pat. No. 4,052,368 to Larson, U.S. Pat. No. 4,413,080 to Blake, and U.S. Pat. No. 4,569,960 also to Blake, each incorporated herein by reference.
A need has existed to provide adhesives for a general purpose label and tape construction which are repulpable and not sensitive to changes in relative humidity and where no special precautions are required for storage or use as an adhesive for a label or tape. We have developed repulpable pressure-sensitive adhesives which contain from about 10 to about 20 percent by weight of a mixture of carboxylic acids typically a mixture of acrylic and methacrylic acids. While functional as a repulpable pressure-sensitive adhesive for a variety of papers, they do not however function with alkaline phosphored paper, a paper of superior whiteness used for postage stamps sold in the United States and elsewhere.
The reason is attributed to the reaction of zinc orthosilicate used to identify the location of a stamp on an envelope, with carboxylic groups in the polymer which essentially deactivate the adhesive. This reaction is amplified on exposure to high humidity.
A repulpable pressure-sensitive adhesive which is compatible with alkaline phosphored papers is the subject matter of the instant invention.